This application claims a priority based on Japanese Patent Application No. 2000-059104 filed on Mar. 3, 2000, the entire contents of which are incorporated herein by reference for all purposes.
1. Field of the Invention
The present invention relates to a mobile communication system and a method of controlling transmission power, and in particular to a base station apparatus, a mobile communication system using the base station apparatus, and a method of controlling transmission power in a cellular mobile communication system utilizing the code division multiple access (hereinafter, referred to as CDMA).
2. Related Art Statement
In the cellular mobile communication system of the CDMA system, a plurality of mobile stations can share the same frequency band to conduct communication with a base station apparatus. However, in this mobile communication system, when a certain mobile station conducts communication with the base station apparatus, a signal (undesired signal) transmitted by another mobile station to the base station apparatus interferes with a signal (desired signal) transmitted by that certain mobile station to the base station apparatus, becoming a disturbance to the communication between the desired mobile station and the base station apparatus. At that time, the interference level becomes larger in proportion to the reception level of the undesired signal received by the base station apparatus. Further, the reception level of the undesired signal is proportional to the transmission power of the transmission from the undesired mobile station. Accordingly, in a mobile communication system of the CDMA system, in order to suppress the interference level to the minimum, it is required that the base station apparatus controls the transmission power of the mobile station, such that the transmission power from the mobile station is received in the base station apparatus always at the necessary but minimum reception level.
As the conventional technique of such a method of controlling transmission power, is known a method called the closed loop power control. A base station apparatus that performs this method of controlling transmission power measures reception quality such as a Signal-to-Interference Ratio (hereinafter, referred to as a SIR value) of a reverse link channel, received power, etc., and compares the measured reception quality with desired target values decided by the system. Then, based on the results of the comparison, the base station apparatus prepares a transmission power control signal for instructing the mobile station to increase or decrease the transmission power, and adds that signal to a frame of transmit data for transmission to the mobile station. On receiving the transmission power control signal, the mobile station controls the transmission power in accordance with the instruction of increasing or decreasing. By repeating this control, the reception quality level at the base station apparatus can be converged to the target values.
Further, in recent years, methods of suppressing interference generated in the course of despreading a code-multiplexed signal for detecting the desired signal are attracting attention. This interference is generated owing to cross-correlation of respective spread codes assigned to mobile stations. As one method of suppressing interference, is proposed an interference suppressor for a multi-user reception system.
In the multi-user reception system, all the user signals are once demodulated in the first stage, to generate an interference replica of each user. Then, the interference replicas of the users other than the user in question are subtracted from the received input signal to suppress the interference. In the next stage, this signal from which the interference is suppressed is used for repeating demodulation of the user signal as the desired signal. By such operation, the signal as the result of the two-stage demodulation is improved in signal quality than the result of the demodulation in the first stage. By using such configuration in multiple stages and by repeating a series of processing a plurality of times, interference can be suppressed.
As such a method of controlling transmission power, is mentioned the technique described in Japanese Unexamined Patent Laid-Open No. 10-247894. According to 10-247894, the received signal is used for measuring each user""s SIR value, after the received signal passes through all the interference suppressing receiving part consisting of multistage interference suppressing receiving circuits. Then, that SIR value and a predetermined target SIR value are compared by a comparator circuit, and the result of comparison is added as transmission power control bits to a transmit frame before transmission to the mobile station.
The above-described conventional technique has a problem that the interference suppressing effect can not be sufficiently obtained.
This will be described in detail. In 10-247894, the control of transmission power is performed using the SIR value of the signal that has passed the interference suppressing receiving circuit in the last stage. As a result, processing delay due to the multi-stage interference suppressing receiving circuits becomes large, and it takes a lot of time to generate a transmission power control signal after receiving the reception signal through an antenna. Accordingly, error in controlling the transmission power becomes large. When, the transmission power becomes larger than the desired level for a receiver, interference with another mobile station increases.
On the other hand, it is considered that, in order to decrease the processing delay, the transmission power control is performed using the SIR value of the signal that has passed the interference suppressing circuit in the first stage. In the case that the SIR value of the signal that has passed the interference suppressing circuit in the first stage is used, time between the reception of the received signal and the measurement of the SIR value can be shortened. However, the output of the interference suppressing circuit in the first stage does not sufficiently reflect the interference suppressing effect. Thus, there arises a case that, even when the communication can be conducted with reduced transmission power of each mobile station within the system, it is not possible to control each mobile station to decrease its transmission power. Further, this method has a problem of inefficiency since transmission power is controlled using a signal that has not undergone sufficient interference suppressing effect.
An object of the invention is to provide a base station apparatus, a mobile communication system, and a method of controlling transmission power, that can solve the above-described problems of the conventional technique, and can control transmission power with better interference suppressing effect and smaller processing delay.
To that end, the present invention provides a base station apparatus in a mobile communication system including a plurality of mobile stations and a plurality of base station apparatuses each having an interference suppressing receiving part containing interference suppressing circuits in a plurality of stages, the base station apparatus comprising: a means for calculating a correction SIR value based on a SIR value measured from a received signal subjected to interference suppression by an interference suppressing receiving circuit in a former stage in the interference suppressing receiving part and an SIR value of the received signal subjected to interference suppression by an interference suppressing receiving circuit in a later stage; a means for estimating a transmission power control SIR value based on the calculated correction SIR value; a means for generating a transmission power control signal using the estimated transmission power control SIR value; and a transmission power control means for adding the generated transmission power control signal to a downlink frame, to control transmission power of a mobile station.
Particularly, the correction SIR value is calculated by subtracting the former stage SIR value from the later stage SIR value, or by taking an average from integral of a difference between the later stage SIR value and the former stage SIR value in a certain cycle.
More particularly, the transmission power control SIR value is calculated by subtracting the correction SIR value from the former stage SIR value; and the transmission power control signal is generated based on a result of comparison between the transmission power control SIR value and a target SIR value.
Further, the former stage SIR value is an SIR value measured based on the received signal outputted from one of interference suppressing receiving circuits in the first stage through a stage preceding the last stage of the interference suppressing receiving part containing interference suppressing circuits in a plurality of stages.
As another configuration, the present invention provides a base station apparatus in a mobile communication system including a plurality of mobile stations and a plurality of base station apparatuses each having an interference suppressing receiving part containing interference suppressing circuits in a plurality of stages, the base station apparatus comprising: a means for calculating a correction SIR value based on a SIR value measured from a received signal subjected to interference suppression by an interference suppressing receiving circuit in a former stage in the interference suppressing receiving part and an SIR value of the received signal subjected to interference suppression by an interference suppressing receiving circuit in a later stage; a means for estimating a corrected target SIR value based on the calculated correction SIR value; a means for generating a transmission power control signal using the estimated correction target SIR value; and a transmission power control means for adding the generated transmission power control signal to a downlink frame, to control transmission power of a mobile station.
Particularly, the correction SIR value is calculated by subtracting the former stage SIR value from the later stage SIR value, or by taking an average from integral of a difference between the later stage SIR value and the former stage SIR value in a certain cycle.
More particularly, the corrected target SIR value is calculated by subtracting the correction SIR value from a target SIR value; and the mentioned transmission power control signal is generated based on a result of comparison between the former stage SIR value and the corrected target SIR value, so that the mentioned cycle of averaging is shorter than a cycle of transmission power control operation between the base station apparatus and a base station controller.
As still another configuration, the present invention provides a mobile communication system comprising a plurality of mobile stations and a plurality of base station apparatuses each having an interference suppressing receiving part containing interference suppressing circuits in a plurality of stages, with each base station apparatus being provided with the above-mentioned means.
Further, the above-described object is attained in the above-described mobile communication system by using a CPU and a software memory in the base station apparatus, in order to implement the means for calculating a correction SIR value and the means for calculating the transmission power control SIR value, or by using a CPU and a software memory, in order to implement the means for calculating the corrected target SIR value.